A vehicular automatic transmission is known that induces a transmission rotational speed from a predetermined map based on a vehicle speed and a load of an engine and automatically performs the gear change operation based on the transmission rotational speed. There has also been known a transmission control device which is configured such that the gear change operation (which brings about changes of an engine rotational speed and an engine output) is restricted when it is determined that the vehicle is turning or cornering.
For example, JP-A-63-72950 discloses a transmission control device which determines whether the vehicle is turning based on at least a vehicle speed and a steering angle of a steering wheel and restricts a shift-down of the automatic transmission. According to this transmission control device, it is possible to restrict the shift-down only when the vehicle is turning in which the vehicle travels with predetermined vehicle speed and steering angle or more.
Further, JP-A-63-207735 discloses an automatic transmission which includes a releasing device which, when a state in which a steering angle assumes a predetermined value or less is continued for a predetermined time or more, releases the restriction of a shift change which is carried out when the cornering is performed as in the case of JP-A-63-72950.
The above-mentioned transmission control device is particularly applicable to a motorcycle which controls a posture of a vehicle body during cornering by adjusting a driving force. For example, as a shift-up during turning typically adversely influences vehicle riding feeling, there may also be a case in which immediately after a throttle is subject to an open operation to impart a larger driving force to stabilize the posture of the vehicle body, a transmission rotational speed reaches the above-mentioned transmission rotational speed thus automatically causing a shift-up. The technique disclosed in JP-A-63-72950 can restrict such a shift-up.
However, the technique in JP-A-63-72950 requires means for detecting a steering angle of a steering wheel and hence, there exists a drawback that a control system for restricting the shift-up during turning becomes complicated. Further, because, for example, a steering angle of the motorcycle during turning is small (e.g., compared to a steering angle of a four-wheeled vehicle during turning), there has been a demand for a system which can determine that the motorcycle is turning by an element other than the steering angle.
Therefore, it is an object of the present invention, for example, to provide an automatic transmission control device which can overcome a task of the above-mentioned related art and can effectively restrict a shift-up during cornering with the simple constitution.